Hydraulic machines such as, for example, dozers, loaders, excavators, motor graders, and other types of heavy equipment use one or more hydraulic actuators to accomplish a variety of tasks. These actuators are fluidly connected to a pump of the machine that provides pressurized fluid to chambers within the actuators. As the pressurized fluid moves into or through the chambers, the pressure of the fluid acts on hydraulic surfaces of the chambers to affect movement of the actuators and a connected work tool. When the pressurized fluid is drained from the chambers it is returned to a low pressure sump of the machine.
One problem associated with this type of hydraulic arrangement involves efficiency. In particular, the fluid draining from the actuator chambers to the sump often has a pressure greater than a pressure of the fluid already within the sump, especially when the actuators are moving in a direction aligned with the pull of gravity (i.e., when actuator movement is being assisted by a weight of the tool and associated load). As a result, the higher pressure fluid draining into the sump still contains some energy that is wasted upon entering the low pressure sump. This wasted energy reduces the efficiency of the hydraulic system.
One method of improving the efficiency of hydraulic machine is described in U.S. Pat. No. 6,467,264 (the '264 patent) issued to Stephenson et al. on Oct. 22, 2002. The '264 patent describes a hydraulic system having a cylinder containing a piston that is connected by a rod to drive a load. The piston divides an internal cavity of the cylinder into a head chamber and a rod chamber, both of which are connected to an array of four bidirectional, proportional control valves. A first control valve controls the flow of hydraulic fluid from a pump to the head chamber. A second control valve regulates the flow of hydraulic fluid between the head chamber and a return line. A third control valve governs the flow of hydraulic fluid from the pump to the rod chamber, and a fourth control valve controls the flow of hydraulic fluid between the rod chamber and the return line. By simultaneously operating different combinations of the control valves, hydraulic fluid from the pump can be applied to one of the cylinder chambers and exhaust to the return line from the other chamber to drive the piston and connected load in one of two directions.
Several modes of operation of the hydraulic system described above are disclosed in the '264 patent, including a regenerative mode of operation. In the regenerative mode of operation, the load acting on the piston tends to extend the rod from the cylinder due to gravity acting on the load. In this mode, the third valve is opened to regulate a reverse flow of fluid exhausting from the rod chamber and control the rate at which the load drops, and the first control valve is opened to meter the flow of fluid into the head chamber. The second and fourth control valves are closed during this mode of operation. Although little or no energy from the pump needs to be exerted to lower the load, additional fluid is still required to fill the expanding head chamber. And, if all of the fluid were provided by only the pump, energy would be wasted in the pressurizing of that fluid. In addition, if the fluid from within the rod chamber were simply exhausted to the return line, energy within that fluid would also be wasted. Thus, the simultaneous opening of the first and third control valves allows fluid from both the rod chamber and the pump to enter the head chamber and help prevent cavitation. Additional modes of regeneration are similarly described in the '264 patent.
Although the '264 patent disclosed operational modes that may help increase machine efficiency, the disclosed modes may be less than optimal. That is, the on/off approach to initiating regeneration (i.e., the opening/closing operations of the valves described above) may result in poor system modulation and jerky response.
The disclosed hydraulic system is directed to overcoming one or more of the problems set forth above and/or other problems of the prior art.